Heating cables are well known, and are used in a wide variety of applications. A typical heating cable conducts electricity, and in doing so dissipates in the form of heat some of the electrical energy which it conducts. The heating cable can be used to heat a pipe to ensure that the contents of the pipe are maintained at a certain temperature, for example above the freezing point of the contents. The heating cable maybe in contact with either the inside or the outside of the pipe, and may extend along the pipe in a linear fashion or be wound around the pipe. Heating cables also have other applications, for example under-floor heating, the heating of car seats and any other application where heating may be required.
In more recent decades, self-regulating heating cables have been designed. These self-regulating heating cables often comprise a material having a positive temperature coefficient of resistance. This means that as the heating cable gets hotter, its resistance increases. Since its resistance increases, the current flow to the cable is reduced, causing the temperature of the cable to reduce in a corresponding manner. Thus, the heating cable self-regulates. An advantage of self-regulating heating cables is their inherent safety properties. For example, self-regulating heating cables cannot overheat or burnout, since the cable can be constructed to reduced the current flow to almost zero at a pre-determined safe temperature (e.g. below the combustion temperatures of materials used to construct the cable or of materials in the environment in which the cable is used).
Most early heating cables were provided with one or more electrical conductors which ran along the length of the heating cable. These earlier heating cables were designed to be used with single-phase electrical power supplies. More recently, heating cables have been designed which take advantage of the benefits of three-phase electrical power supplies. For instance, single-phase heating cables can have circuit lengths of a few hundred meters, whereas three-phase heating cables can have circuit lengths of many kilometers.
Single-phase heating cables can either be constant power or self-regulating. However, existing three-phase heating cables are only constant power.